Magneto-optic recording systems comprise an optical read/write beam and a magnetizable storage medium, usually a disk. Writing is accomplished by a high intensity focused light beam, such as a laser, which alters the magnetization of the medium by heating a localized area of the medium above its Curie temperature and allowing the area to cool under an applied magnetic field. Reading is accomplished by a lower intensity plane-polarized beam which, upon transmission through and/or reflection from the medium, experiences a Kerr rotation in polarization through a characteristic angle .theta. or -.theta. depending on the local magnetization of the medium. Optical detectors may be used to translate the Kerr rotation angle into a binary data signal.
One method of increasing information storage on a magneto-optic medium is to increase the number of independent recording layers in the medium. The recording layers are designed to have different writing temperatures by varying their magnetic coercivity, H.sub.c, and/or Curie temperature so that a different power laser write pulse will record on only one, two, or more layers.
This method, however, usually results in a diminished read signal from the magneto-optic medium, especially where, for example, the signal is measured by the reflected light (Kerr rotation, .theta.). Thus, when individual layers are independently switched within the medium, the resulting magneto-optic signal is only a fraction of the signal typically received from single-layer media.
For example, a single-layer medium can be recorded in either an "up" state, where the magneto-optic layer is magnetized upward, or a "down" state, where the magneto-optic layer is magnetized in the opposite direction, giving rise to two possible states. If the "up" state has a magneto-optic (or Kerr) rotation of +1.degree., then the "down" state has a Kerr rotation of -1.degree., and the difference between the two states, i.e., the signal, is 2.degree..
In the case of a two-layer medium, the magneto-optic layer can be recorded in three different states: "up-up," where both layers are aligned upward; "down-down," where both layers are aligned downward; and "up-down," wherein each layer is magnetized in the opposite direction. (Note that "up-down" is equivalent to "down-up" because in both cases the signals from the two layers cancel each other out.) Thus, the ability to utilize three magnetic states, instead of two for the single-layer medium, leads to an increase in storage density of 50%.
The increased storage density, however, has a drawback: decreased signal. If the up-up state has a magneto-optic rotation of +1.degree., and the down-down state has a rotation of -1.degree., then the difference between those two states is 2.degree., just as it was for the single-layer magneto-optic medium. However, the difference between the up-up (+1.degree.) or down-down (-1.degree.) states and the up-down (0.degree.) states is only 1.degree., which is only half of the 2.degree. separation of the single-layer magneto-optic medium. This results in a loss of signal of 6 dB.